Gas turbine engines are known to include a compressor section, a combustor section, and a turbine section. Many components that form the turbine section, such as the stationary vanes, rotating blades and surrounding ring segments, are directly exposed to hot combustion gasses that can exceed 1500 degrees C. and travel at velocities approaching the speed of sound. The rotating blades and stationary vanes are arranged circumferentially in rows with each row being comprised of a plurality of blades and vanes. To help shield the turbine components from this extreme and damaging operating environment, they are often coated with ceramic thermal barrier coating materials, such as ytrria stabilized zirconia oxide (YSZ).
However, thermal barrier coatings tend to chip, delaminate, or spall from the underlying turbine component during operational service life, thereby causing a damaged turbine component. Moreover, the spalled thermal barrier coating itself can constitute a harmful foreign object within the gas path that damages other turbine components.
In the past, inspection for damage to turbine components has been performed by partially disassembling the gas turbine engine and performing visual inspections on individual components. In-situ visual inspections may be performed without engine disassembly by using a borescope inserted into a gas turbine engine, but such procedures are labor intensive, time consuming, costly, and require that the gas turbine engine be shut down.
Due to the strong economic incentive to inspect for turbine component damage while the gas turbine is operating, various on-line and real-time methods and apparatus for detecting and locating defects in turbine components while the turbine engine is in operation have been proposed, including acoustic, optical and infrared. However, each of these methods and apparatus have appreciable disadvantages.
Accordingly, there continues to be a need for methods and apparatus for the on-line and/or real-time detection of damage to turbine components.